The present invention generally relates to a fluid bladder with a check valve and, more specifically, to a bladder wherein the check valve is formed of a mating pair of plastic sheets heat welded in substantially parallel lines to form a gap-free channel.
In the past, inflatable air bladders have been formed with integral valves. A common type of valve used for inflatable air bladders is a "duckbill" type valve. This type of valve may be formed of two parallel flaps joined along the edges to form a collapsed tube. The tube extends into the bladder, with a slightly open inlet or mouth for inflation.
In order to inflate the air bladder, air is forced into the bladder through the mouth of the valve. Thus, the air pressure is greater outside the air bladder than inside the air bladder. This pressure differential causes the flaps of the valve to separate and the valve to open.
After the air bladder is inflated, the air pressure inside the air bladder is greater than the air pressure outside the bladder. The greater air pressure inside the bladder forces the flaps of the valve together. Thus, the valve is sealed in a closed position, and the air inside the bladder cannot escape.
In order for this type of valve to function properly, the flaps must be in contact with each other and in a gap-free relationship in the absence of any pressure differential across the openings of the valve. Therefore, when the air bladder is deflated, and no pressure is being either forced into or out of the air bladder, the flaps must be in gap-free contact with each other. Any gap between the flaps of the valve will prevent the valve from sealing and will cause the valve to leak air when inflated.
The gaps in duckbill valves may be a result of the method by which the two flaps are joined together to form the duckbill valve. Typically, the two flaps are joined together along the edges by a conventional welding process. A conventional radio frequency (RF) welding die has brass electrodes mounted to and protruding from a supporting base of conducting or insulating material. As the sheet plastic is melted in the vicinity of the weld, the brass electrode sinks into the work. The flat surface of a platen or a buffer sheet maintains the opposite plastic sheet flat. The exerted pressure of the platen or buffer sheet causes the molten plastic to flow between the sheets of plastic and form beads at the boundary of the weld between the sheets. This distortion in the work at the weld becomes worse as thicker sheet plastic material is used because the electrodes sink further in, displacing more molten plastic. Further, lack of support of the plastic sheet material being welded causes the heated plastic to sag, leaving a gap in the valve channel.
A variation on a valve for use with an air bladder is disclosed in Shabram, U.S. Pat. No. 3,230,663. Shabram teaches an integral valve structure in which two plastic sheets are first heat-sealed together to form the bladder, then extensions of the plastic sheets folded inwardly into the bladder to form the valve. When the air bladder is inflated, the air pressure pushes against the inwardly-folded extensions, forcing them into closed contact with each other. Although this structure and method of fabrication is simple, the valve often fails to seal, thereby rendering the valve unreliable.
Attempts have been made to overcome the unreliability of this type of valve. For example, Johnson, U.S. Pat. No. 4,287,920, discloses a self-sealing duckbill valve. The valve is similar to the valve disclosed in Shabram, except that a tacky substance is applied between the flaps to enhance the adhesion of the flaps when there is a pressure differential across the openings. A reservoir of this tacky material is provided in the valve. However, similar to Shabram, if the tacky substance is not used, the valve also proves to leak and therefore unreliable.
Accordingly, a principal object of the present invention is to provide a simple and reliable sheet plastic check valve for air bladders or the like, which does not leak air and a method for making such valves.